Quantum cascade laser

ABSTRACT

A quantum cascade laser  1 , which generates infrared light or other light of a predetermined wavelength by making use of intersubband transitions in a quantum well structure, is arranged by forming, on a GaAs substrate  10 , an AlGaAs/GaAs active layer  11  having a cascade structure in which quantum well light emitting layers and injection layers are laminated alternately. Also, at the GaAs substrate  10  side and the side opposite the GaAs substrate  10  side of active layer  11 , is provided a waveguide structure, comprising waveguide core layers  12  and  14 , each being formed of an n-type GaInNAs layer, which is a group III-V compound semiconductor that contains N (nitrogen), formed so as to be lattice matched with the GaAs substrate  10 , and waveguide clad layers  13  and  15 , each formed of an n ++ -type GaAs layer. A quantum cascade laser, with which the waveguide loss of generated light in the laser is reduced, is thereby realized.

This is a divisional application of application Ser. No. 10/776,615,filed on Feb. 12, 2004 now U.S. Pat. No. 7,359,418, which isincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns a quantum cascade laser that makes use ofintersubband transitions in a quantum well structure.

2. Related Background Art

Light of the middle infrared wavelength range (for example, wavelengthsof 3 to 10 μm) is deemed important in various fields, such as medicalfields, biomedical measurement, environmental measurement, etc. Thereare thus increasing needs for a high-performance coherent light sourcefor this wavelength range.

However, a laser light source using normal interband transitionsoperates only at low temperature in the middle infrared range, and alaser light source capable of operation under room temperature,continuous (CW) emission operation, or high output operation, etc., hasnot been realized. The realization of a high-performance laser lightsource that can take the place of carbon dioxide gas lasers, which havebeen used since priorly as coherent light sources for the infraredrange, solid-state lasers, which use wavelength conversion opticalcrystals, free-electron lasers, which accompany large-scale facilities,etc., (see for example, the literature, C. Sirtori et al., “Low-lossAl-free waveguides for unipolar semiconductor lasers”, Appl. Phys. Lett.vol. 75 (1999) pp. 3911-3913.) is to be hoped.

SUMMARY OF THE INVENTION

The development of quantum cascade lasers as infrared light emittingelements making use of semiconductor materials has been progressing inrecent years. A quantum cascade laser is a laser light source thatgenerates light by making use of intersubband transitions in a quantumwell structure with low temperature dependence, and, in principle, cangenerate light in the wavelength range of approximately 3 to 70 μm.

Also with a quantum cascade laser, a high output is enabled by a cascadestructure in which quantum well light emitting layers are disposed inmultiple layers. A quantum cascade laser furthermore has great potentialas an infrared coherent light source due to being a unipolar device thatmakes use of intersubband transitions as mentioned above and not havinga PN junction, being able to generate ultrashort pulse light at afrequency response of high speed, being small in relaxation oscillation,enabling multiple wavelength emission and broadband emission, and beingexcellent in temperature characteristics.

As such a quantum well cascade laser, a quantum cascade laser (InP-QCL)using an InP substrate as the semiconductor substrate is mainly known. Aquantum cascade laser (GaAs-QCL) that uses a GaAs substrate, which ismore inexpensive and better in crystal properties than InP, has alsobeen developed. For example, a quantum cascade laser (GaAs-QCL) thatuses a GaAs substrate as the semiconductor material of the substrate isdisclosed in the abovementioned literature. With the GaAs-QCL describedin this literature, the active layer is arranged from a GaAs/AlGaAslayer, and between the GaAs substrate and the active layer, a waveguideclad layer (n⁺⁺-type GaAs layer) and a waveguide core layer (n-type GaAslayer) are disposed in that order from the GaAs substrate side. It isreported that laser emission of a wavelength of 9 μm was achieved at atemperature of 77K by this arrangement.

When such a quantum cascade laser is to be used as an infrared laserlight source, the optical loss due to waveguide loss within the laser,etc., becomes a problem since the infrared light that is generated is oflong wavelength. That is, in a waveguide structure inside a laser inwhich infrared light is guided, the light undergoes free carrierabsorption at an absorption coefficient α, expressed by the followingequation.α=Ne ²λ²/(πnc ³ m*τ)In this equation, N is the carrier density, e is the unit charge, λ isthe wavelength of light, n is the refractive index, c is the speed oflight, m* is the effective mass of an electron inside the waveguide, andτ is the relaxation time.

As can be understood from this equation, the free carrier absorptionthat occurs in the waveguide structure inside a laser increases inproportion with the square of the wavelength λ of light. The waveguideloss of generated light, due to free carrier absorption inside thelaser, is thus problem in a quantum cascade laser that generatesinfrared light of a long wavelength.

This invention has been made to resolve the above problem and an objectthereof is to provide a quantum cascade laser, with which the waveguideloss of generated light within the laser is lessened.

In order to achieve the above object, a quantum cascade laser by thisinvention comprises: (1) an active layer, having a cascade structure, inwhich quantum well light emitting layers and injection layers arelaminated alternately on a semiconductor substrate formed of GaAs, andgenerating light by intersubband transitions in a quantum wellstructure; (2) a waveguide core layer, formed adjacent the active layer;and (3) a waveguide clad layer, formed adjacent the waveguide core layerat the side opposite the side of the active layer; and wherein (4) thewaveguide core layer is formed of a group III-V compound semiconductor,containing, as the group V elements, N and at least one element selectedfrom the group consisting of As, P, and Sb, and formed so as to belattice matched with the semiconductor substrate.

This invention also provides a quantum cascade laser comprising: (1) anactive layer, having a cascade structure, in which quantum well lightemitting layers and injection layers are laminated alternately on asemiconductor substrate formed of InP, and generating light byintersubband transitions in a quantum well structure; (2) a waveguidecore layer, formed adjacent the active layer; and (3) a waveguide cladlayer, formed adjacent the waveguide core layer at the side opposite theside of the active layer; and wherein (4) the waveguide core layer isformed of a group III-V compound semiconductor, containing, as the groupV elements, N and at least one element selected from the groupconsisting of As, P, and Sb, and formed so as to be lattice matched withthe semiconductor substrate.

With the above-described quantum cascade laser, in the waveguidestructure for the active layer in which quantum well light emittinglayers are disposed in multiple stages with injection layers in between,a waveguide core layer, for guiding the infrared light or other lightgenerated inside the laser, is formed using a group III-V compoundsemiconductor, containing N (nitrogen) as the group V element.

With an arrangement in which the core layer is formed of such asemiconductor material, the waveguide loss of generated light, due tofree carrier absorption inside the laser, is reduced. Also with thiswaveguide structure, the effective refractive index of the waveguidecore layer increases. The thickness of the waveguide core layer and cladlayer necessary for light containment can thereby be made thin.

Here, the waveguide core layer is preferably formed to a predeterminedthickness that is set so that optical modes of higher orders will not beguided. By doing so, the light generated inside the laser can be guidedand output satisfactorily.

Also, the waveguide clad layer preferably contains a high-concentrationdoped layer formed of a group III-V compound semiconductor, containing,as the group V elements, N and at least one element selected from thegroup consisting of As, P, and Sb. The light generated inside the laseris thereby restrained from leaking to the plasmon mode.

This invention also provides a quantum cascade laser comprising: asemiconductor substrate formed of GaAs; and an active layer, disposed onthe semiconductor substrate and having a plurality of quantum well lightemitting layers, generating light by means of intersubband transitionsin a quantum well structure, and a plurality of injection layers,respectively disposed between the plurality of quantum well lightemitting layers and forming a cascade structure along with the quantumwell light emitting layers; and wherein the quantum well light emittinglayers and the injection layers of the active layer are formed tocontain group III-V compound semiconductors, each containing, as thegroup V elements, N (nitrogen) and at least one element selected fromthe group consisting of As, P, and Sb.

In the case of a GaAs-QCL, which uses a GaAs substrate as thesemiconductor substrate, ultrafine processing using a processtechnology, such as dry etching, etc., can be applied and thus evenhigher performance and higher functions can be anticipated. A GaAssubstrate is also excellent in terms of cost since inexpensive rawmaterials can be used. Also with regard to pulsed operation, GaAs-QCL'sfar surpasses InP-QCL's in peak output. GaAs-QCL's thus have variousexcellent characteristics over InP-QCL's and the making of GaAs-QCL'shigh in performance (for example, the realization of continuous (CW)emission at high temperature) is being desired.

Meanwhile, a semiconductor laser that emits light of a wavelength in theinfrared range generally requires a cooling device. This is becauseenergy gap transitions that are smaller than those of a semiconductorlaser that emits light of a wavelength in the visible range are used andthe sensitivity to the heat distribution of carriers is thus extremelyhigh. Even a quantum cascade laser requires cooling for CW operation andthus requires a cooling device. Cooling becomes especially necessarywhen the self-heating by the element itself is high.

Here, with the prior-art GaAs-QCL arrangement described in theabovementioned literature, in order to reduce the free carrierabsorption inside the waveguide clad layer and the waveguide loss insidethe laser due thereto, the intensity of light must be attenuatedadequately before the waveguide clad layer is reached and thus thethickness of the waveguide core layer must be made adequately thick.There is thus the problem that self-heating becomes high due toincreased element resistance of a GaAs-QCL in the case where theGaAs-QCL is to perform CW emission at a high temperature, etc.

On the other hand, with the quantum cascade laser of the above-describedarrangement, the effective refractive index of the active layer can bemade greater than that of the semiconductor substrate formed of GaAs.The semiconductor substrate can thereby be made to function as awaveguide clad layer and the active layer can be made to function as awaveguide core layer. The waveguide loss of generated light, due to freecarrier absorption inside the laser, can thus be reduced by such anarrangement. Also in this case, since the thickness of the element canbe made thin, the element resistance can be reduced and self-heating ofthe element due to the element resistance can be restrained. The layerstructure between the semiconductor substrate and the active layer canalso be simplified.

With the above-described quantum cascade laser, the composition ratio ofN in the group III-V compound semiconductor is preferably no less than0.1% and no more than 40%. By the composition ratio of nitrogen being noless than 0.1%, the containment of light in the active layer can be madedefinite. Also, by the composition ratio of nitrogen being no more than40%, the band gap can be made small.

Furthermore, the above-described cascade laser is preferably equippedwith a semiconductor layer formed adjacent the active layer, disposed atleast either between the semiconductor substrate and the active layer orat the side of the active layer opposite the semiconductor substrateside and formed of a group III-V compound semiconductor, containing, asthe group V elements, N and at least one element selected from the groupconsisting of As, P, and Sb. In this case, the abovementionedsemiconductor layer and the active layer can be made to function as awhole as a core layer. And since the effective refractive index of thiscore layer can be made greater than the refractive index of thesemiconductor substrate formed of GaAs or furthermore greater than theeffective refractive index of the active layer, light containment can bestrengthened further.

The present invention will be more fully understood from the detaileddescription given hereinbelow and the accompanying drawings, which aregiven by way of illustration only and are not be considered as limitingthe present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view showing the arrangement of a firstembodiment of a quantum cascade laser.

FIG. 2 is a diagram showing a cascade structure and intersubbandtransitions in a quantum well structure of an active layer.

FIG. 3 is a sectional side view showing an example of the arrangement ofa prior-art quantum cascade laser.

FIG. 4 is a sectional side view showing the arrangement of a secondembodiment of a quantum cascade laser.

FIG. 5 is a sectional side view showing the arrangement of a thirdembodiment of a quantum cascade laser.

FIG. 6 is a sectional side view showing the arrangement of a fourthembodiment of a quantum cascade laser.

FIG. 7 is a sectional side view showing the arrangement of a fifthembodiment of a quantum cascade laser.

FIG. 8 is a schematic diagram showing an example of a cascade structureand intersubband transitions in a quantum well structure of an activelayer.

FIG. 9 is a table showing an example of the thickness, order oflamination, and carrier concentrations of the respective layers of thelaminated units that form an active layer.

FIG. 10 is a diagram showing the refractive index distribution and thelight intensity distribution of the quantum cascade laser of FIG. 7.

FIG. 11 is a sectional side view showing an example of the arrangementof a prior-art quantum cascade laser.

FIG. 12 is a table showing an example of the thickness, order oflamination, and carrier concentrations of the respective layers of thelaminated units that form an active layer.

FIG. 13 is a diagram showing the refractive index distribution and thelight intensity distribution of the quantum cascade laser of FIG. 11.

FIG. 14 is a diagram showing the relationship of the composition ratioof nitrogen in the GaInNAs, which forms the active layer, and the lightcontainment factor.

FIG. 15 is a schematic diagram of an example of the conduction banddiscontinuity between a quantum well layer and quantum barrier layers ina prior-art quantum cascade laser.

FIG. 16 is a schematic diagram of an example of the conduction banddiscontinuity between a quantum well layer and quantum barrier layers inthe quantum cascade laser of the fifth embodiment.

FIG. 17 is a sectional side view showing the arrangement of a sixthembodiment of a quantum cascade laser.

FIG. 18 is a diagram showing the refractive index distribution and thelight intensity distribution of the quantum cascade laser of FIG. 17.

FIG. 19 is a diagram showing the relationship of the thickness of theGaAs layer and the light containment factor of the quantum cascade laserof FIG. 17.

FIG. 20 is a sectional side view showing the arrangement of a seventhembodiment of a quantum cascade laser.

FIG. 21 is a diagram showing the refractive index distribution and thelight intensity distribution of the quantum cascade laser of FIG. 20.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of this invention's quantum cascade laser shallnow be described in detail along with the drawings. In the descriptionof the drawings, the same elements shall be provided with the samesymbols and redundant description shall be omitted. The dimensions andproportions of the drawings may not necessarily match those of thedescription.

FIG. 1 is a sectional side view showing the arrangement of a firstembodiment of this invention's quantum cascade laser. Laser 1, shown inFIG. 1, is an AlGaAs/GaAs type quantum cascade laser (GaAs-QCL) thatuses GaAs as the semiconductor material of the substrate.

Quantum cascade laser 1 comprises a GaAs substrate (semiconductorsubstrate) 10, and the respective semiconductor layers of an activelayer 11, waveguide core layers 12 and 14, and waveguide clad layers 13and 15, which are formed on GaAs substrate 10. Also, of the sidesurfaces of quantum cascade laser 1, mirror surfaces, which form theoptical resonator of this laser 1, are formed on two predeterminedsurfaces that oppose each other.

Active layer 11 is a semiconductor layer that is formed on GaAssubstrate 10 and generates light of a predetermined wavelength (forexample, light in the middle infrared wavelength range) by making use ofintersubband transitions in a quantum well structure. In the presentembodiment, in correspondence to the use of GaAs substrate 10 as thesemiconductor substrate as mentioned above, active layer 11 is arrangedas an AlGaAs/GaAs multiple quantum well structure that uses GaAs inquantum well layers and uses AlGaAs in quantum barrier layers.

Specifically, active layer 11 is formed, by quantum well light emittinglayers (quantum well active layers) and injection layers being laminatedalternately, to have a cascade structure in which the quantum well lightemitting layers are disposed in multiple stages. The number of quantumwell light emitting layers and injection layers laminated are setsuitably and is, for example, approximately a few hundred.

FIG. 2 is a diagram showing the cascade structure and the intersubbandtransitions in the quantum well structure of the active layer. For thesake of description, in FIG. 2, among the laminated structure of thequantum well light emitting layers and injection layers that form activelayer 11, the cascade structure of three layers, that is, the twoquantum well light emitting layers 102 and 104 and the single injectionlayer 103, sandwiched between light emitting layers 102 and 104, isshown.

As shown in FIG. 2, each of quantum well light emitting layers 102 and104 is formed of quantum well layers 110 and quantum barrier layers 111.In each of light emitting layers 102 and 104, quantum levels n=1, 2, and3 are formed by these quantum well layers 110 and quantum barrier layers111.

Also, injection layer 103 is disposed between light emitting layers 102and 104. This injection layer 103 is formed of quantum well layers 112and quantum barrier layers 113, and quantum well layers 112 are formedto become narrower in width in the direction from light emitting layer102 to light emitting layer 104.

When a bias voltage is applied to quantum cascade laser 1, having activelayer 11 with such a conduction band quantum level structure, electrons101 become injected selectively into the quantum level of n=3 of lightemitting layer 102. An electron 101 that is injected into the quantumlevel of n=3 transits to the quantum level of n=2, and in this process,light hν of a wavelength corresponding to the energy level differencebetween the quantum levels of n=2 and 3 is generated.

Also, the electron 101 that has transited to the quantum level of n=2passes from the quantum level of n=2 through injection layer 103 and isselectively injected into the quantum level of n=3 of light emittinglayer 104. This electron 101 then transits from the quantum level of n=3to the quantum level of n=2 in light emitting layer 104. In thisprocess, light hν of a wavelength corresponding to the energy leveldifference between the quantum levels of n=2 and 3 is generated.

When for example the energy gap between the quantum level of n=3 and thequantum level of n=2 is approximately 300 meV in both cases, thewavelength of the light hν that is emitted from each of light emittinglayers 102 and 104 will be of the middle infrared wavelength range.Light of such wavelength is resonated by the optical resonator of laser1 and output as laser light of a predetermined wavelength. When aplurality of such quantum well light emitting layers and injectionlayers are laminated alternately, electrons move successively in acascading manner among the quantum well light emitting layers and lighthν is generated in the process of the intersubband transition at eachlight emitting layer.

For active layer 11, having the cascade structure shown in FIG. 2,n-type waveguide core layers 12 and 14, which are low-concentrationdoped layers, and n⁺⁺-type waveguide clad layers 13 and 15, which arehigh concentration doped layers, are disposed to form a waveguidestructure by which the light generated in active layer 11 is guidedinside laser 1.

At the GaAs substrate 10 side (lower side in the Figure) of active layer11, waveguide core layer 12 and waveguide clad layer 13 are formed inthat order from the active layer 11 side. Waveguide core layer 12 is asemiconductor layer formed of an n-type GaInNAs layer and is formed tobe adjacent to active layer 11 and be lattice matched with GaAssubstrate 10.

Waveguide clad layer 13 is a semiconductor layer formed of an n⁺⁺-typeGaAs layer and is formed adjacent waveguide core layer 12 at the sideopposite the side of active layer 11. With such an arrangement, therefractive index of waveguide core layer 12 is higher than those of theadjacent active layer 11 and waveguide clad layer 13. A waveguidestructure comprising core layer 12 and clad layer 13 is thus arranged.

Also at the side opposite the GaAs substrate 10 side (upper side in theFigure) of active layer 11, waveguide core layer 14 and waveguide cladlayer 15 are formed in that order from the active layer 11 side.Waveguide core layer 14 is a semiconductor layer formed of an n-typeGaInNAs layer and is formed to be adjacent to active layer 11 and belattice matched with GaAs substrate 10.

Waveguide clad layer 15 is a semiconductor layer formed of an n⁺⁺-typeGaAs layer and is formed adjacent waveguide core layer 14 at the sideopposite the side of active layer 11. With such an arrangement, therefractive index of waveguide core layer 14 is higher than those of theadjacent active layer 11 and waveguide clad layer 15. A waveguidestructure comprising core layer 14 and clad layer 15 is thus arranged.

A contact layer 16 is furthermore formed at the upper side of waveguideclad layer 15. By the above, quantum cascade laser 1, in which n⁺⁺-typeGaAs clad layer 13, n-type GaInNAs core layer 12, AlGaAs/GaAs activelayer 11, n-type GaInNAs core layer 14, n⁺⁺-type GaAs clad layer 15, andcontact layer 16 are laminated in that order onto GaAs substrate 10, isarranged. The typical thickness and carrier concentrations of therespective semiconductor layers are as follows.

TABLE 1 Carrier Thickness concentration n⁺⁺-type GaAs 1 μm n = 6 × 10¹⁸cm⁻³ waveguide clad layer 15 n-type GaInNAs 2.20 μm n = 4 × 10¹⁶ cm⁻³waveguide core layer 14 AlGaAs/GaAs active 1.63 μm Undope layer 11n-type GaInNAs 2.20 μm n = 4 × 10¹⁶ cm⁻³ waveguide core layer 12n⁺⁺-type GaAs 1 μm n = 6 × 10¹⁸ cm⁻³ waveguide clad layer 13

The effects of the quantum cascade laser of the above-describedembodiment shall now be described.

With quantum cascade laser 1 shown in FIG. 1, active layer 11, having acascade structure in which quantum well light emitting layers aredisposed in multiple stages with injection layers in between, isprovided with the waveguide structure wherein GaInNAs, which is a groupIII-V compound semiconductor containing N (nitrogen) as the group Velement, is used to form waveguide core layers 12 and 14, which areadjacent active layer 11 and in which infrared light or other lightgenerated inside laser 1 is guided. With an arrangement in which corelayers 12 and 14 are formed from such a semiconductor material, thewaveguide loss of generated light, due to free carrier absorption insidelaser 1, is reduced.

FIG. 3 is a sectional side view showing an example of the arrangement ofa prior-art quantum cascade laser. Laser 9, which is shown in FIG. 3, isan AlGaAs/GaAs type quantum cascade laser that uses a GaAs substrate.

Quantum cascade laser 9 is arranged by laminating an n⁺⁺-type GaAs cladlayer 93, an n-type GaAs core layer 92, an AlGaAs/GaAs active layer 91,an n-type GaAs core layer 94, an n⁺⁺-type GaAs clad layer 95, and acontact layer 96 in that order onto a GaAs substrate 90.

Unlike quantum cascade laser 1, shown in FIG. 1, with quantum cascadelaser 9, GaAs is used as the semiconductor material of waveguide corelayers 92 and 94, which are adjacent active layer 91. Also, though thetypical thickness and carrier concentrations of the respectivesemiconductor layers are substantially the same as those shown in Table1 for quantum cascade laser 1, with the present example, the thicknessof each of n-type GaAs core layers 92 and 94 is set to a somewhat thickvalue of 3.5 μm.

With quantum cascade laser 9 with such an arrangement, waveguide loss oflight generated in active layer 91, due to free carrier absorptioninside waveguide core layers 92 and 94, becomes a problem, and forexample, continuous emission under room temperature has not beenachieved even with the quantum cascade laser described in the literatureof C. Sirtori et al. Here, as indicated by the equation of theabsorption coefficient α, the free carrier absorption in the waveguidestructure inside a laser decreases in inverse proportion to theeffective mass of an electron inside the waveguide. Thus in order toreduce the waveguide loss due to free carrier absorption, it ispreferable to use a semiconductor material that will make the effectiveelectron mass large in the waveguide core layers.

Meanwhile, with quantum cascade laser 1 shown in FIG. 1, GaInNAs, whichcontains N and is lattice matched with GaAs, is used as thesemiconductor material of waveguide core layers 12 and 14. The effectiveelectron masses and the light absorption coefficients in a waveguidecore layer using GaAs and in a waveguide core layer using GaInNAs withan added amount of N of 1.5% are shown in Table 2.

TABLE 2 GaAs GaInNAs, N = 1.5% Effective mass 0.067 × m₀ 0.097 × m₀Absorption coefficient 1 (Unity) 0.69

Here, m₀ indicates the mass of a free electron. Also, absorptioncoefficients are shown with the absorption coefficient of the waveguidecore layer using GaAs being set to 1. As shown in Table 2, the effectiveelectron mass inside a waveguide is made greater with the arrangement inwhich a group III-V compound semiconductor, containing N, is used as thesemiconductor material of the waveguide core layers in a GaAs-QCL thatuses a GaAs substrate.

In this case, the free carrier absorption inside the waveguide corelayers is made small and the waveguide loss of generated light insidethe laser is reduced. With the example shown in Table 2 in which theadded amount of N is set to 1.5%, the light absorption coefficient dueto free carrier absorption in the GaInNAs waveguide core layer isreduced to 0.69 times that of the prior-art GaAs waveguide core layer.

Also with a quantum cascade laser that generates infrared light, etc.,since the light that is generated is of a long wavelength, lightcontainment inside the laser is difficult and it has been noted that thewaveguide core layers and clad layers must be made thick in order toobtain an adequate light containment effect. However, when the waveguidelayers are thus made thick, the serial resistance of the laser elementincreases. This increase of serial resistance becomes a problemespecially with infrared semiconductor lasers, which often requirecooling.

Meanwhile, the N-containing GaInNAs, which is used in waveguide corelayers 12 and 14 of quantum cascade laser 1 shown in FIG. 1, is asemiconductor material that is greater in refractive index than GaAs.The refractive indices and thickness (μm) required for adequate lightcontainment are shown in Table 3 for a waveguide core layer using GaAsand a waveguide core layer using GaInNAs with an added amount of N of1.5%.

TABLE 3 GaAs GaInNAs, N = 1.5% Refractive index 3.265 3.45 Requiredthickness (μm) 3.50 2.20

As shown in this Table 3, by replacing the GaAs, used priorly in thewaveguide core layers of a GaAs-QCL, by GaInNAs, the effectiverefractive index of the core layer is increased. The thickness of thewaveguide core layers and clad layers required for adequate lightcontainment can thus be made thin. With the example shown in Table 3 inwhich the added amount of N is set to 1.5%, the core layer thickness ofthe GaInNAs waveguide core layer is reduced to 0.63 times that of theprior-art GaAs waveguide core layer. Also, by such reduction of the corelayer thickness, the serial resistance of the laser element is reduced.

Furthermore with regard to the thickness of such waveguide core layers12 and 14, waveguide core layers 12 and 14 are preferably formed to apredetermined thickness that has been set so that optical modes ofhigher orders will not be guided. By doing so, the light generatedinside the laser can be guided and output satisfactorily.

In general, the waveguide core layers applied to a quantum cascade laserarranged using a GaAs substrate are preferably formed of a group III-Vcompound semiconductor, containing N and at least one element among As,P, and Sb as the group V elements, and formed so as to be latticematched with the GaAs substrate. By such an arrangement, the waveguideloss due to free carrier absorption can be reduced as described aboveand the core layer thickness required for light containment can bereduced.

With regard to the amount of N added to the group III-V compoundsemiconductor used in the waveguide core layers, an added amount of 10%or less is preferable for keeping the characteristics of the wave guidecore layers favorable.

As described above, with quantum cascade laser 1 of the arrangementshown in FIG. 1, since the free carrier absorption inside the waveguidecore layers is reduced and the waveguide core layers can be made thin,the temperature characteristics of the laser element is improved andoperation at a high temperature (for example, room temperature) is madepossible. Also, since the waveguide core layers made low in loss, aquantum cascade laser of high output is realized.

Also by the thinning of the waveguide core layers, the elementresistance is reduced. Continuous (CW) emission at a high temperature isthus also enabled. Such effects are furthermore enhanced as the lightgenerated by the laser becomes longer in wavelength. The realization ofa laser light source of even longer wavelength range, for example, ahigh-output quantum cascade laser of the terahertz band is thus alsomade possible.

Also with quantum cascade laser 1, shown in FIG. 1, n⁺⁺-typehigh-concentration doped layers are used as waveguide clad layers 13 and15. By arranging a waveguide clad layer to contain such ahigh-concentration doped layer, the real part of the refractive index ofthe waveguide clad layer is reduced significantly, thereby enablingimprovement of the light containment effect in the waveguide structure.By such improvement of the light containment effect, high-loss leakageof light into the plasmon mode, which occurs at a metalelectrode—semiconductor interface, etc., can be restrained.

Quantum cascade laser 1, having the arrangement shown in FIG. 1, may forexample be prepared using a gas source MBE (Molecular Beam Epitaxy)method. With this method, n⁺⁺-type GaAs clad layer 13, n-type GaInNAscore layer 12, which is lattice matched with GaAs substrate 10,AlGaAs/GaAs active layer 11, in which quantum well light emitting layersand injection layers are laminated, n-type GaInNAs core layer 14, whichis lattice matched with GaAs substrate 10, and n⁺⁺-type GaAs clad layer15 can be grown successively on top of GaAs substrate 10 to form thelaminated structure of quantum cascade laser 1.

Here, though the GaAs layer and the AlGaAs layer may be grown at aconventional growth temperature, that is, at around 600° C., the GaInNAslayer is preferably grown at approximately 450° C. in consideration ofthe phase separation of In. Also, the high-concentration doped GaAslayers that are to become waveguide clad layers 13 and 15 may be formedusing Si, for example, as the dopant.

FIG. 4 is a sectional side view showing the arrangement of a secondembodiment of this invention's quantum cascade laser. As with laser 1shown in FIG. 1, laser 2, shown in FIG. 4, is an AlGaAs/GaAs typequantum cascade laser that uses a GaAs substrate.

Quantum cascade laser 2 is arranged by laminating an n⁺⁺-type GaInNAsclad layer 23, n-type GaInNAs core layer 12, AlGaAs/GaAs active layer11, n-type GaInNAs core layer 14, n⁺⁺-type GaInNAs clad layer 25, andcontact layer 16 in that order onto GaAs substrate 10.

This quantum cascade laser 2 has substantially the same arrangement asquantum cascade laser 1 shown in FIG. 1. However, n⁺⁺-type GaInNAslayers 23 and 25 are used in place of n⁺⁺-type GaAs layers as thewaveguide clad layers adjacent the waveguide core layers 12 and 14. Thetypical thickness and carrier concentrations of the respectivesemiconductor layers are the same as those shown in Table 1 for quantumcascade laser 1.

High-concentration doped layers, having the same N-containing groupIII-V compound semiconductor as that of waveguide core layers 12 and 14as the semiconductor material, may thus be used as the waveguide cladlayers. Here, the high-concentration doped layer in the waveguide cladlayer is effective for improving the light containment effect in thewaveguide structure and restraining the leakage of light into theplasmon mode as mentioned above. On the other hand, such ahigh-concentration doped layer may be a cause of increase of freecarrier absorption.

However, an N-containing semiconductor material, such as GaInNAs, hasthe property that the effective electron mass inside a GaInNAs layerincreases rapidly in accordance with the doping quantity. Thus bychanging the waveguide clad layers from GaAs layers to GaInNAs layers,further reduction of the free carrier absorption is enabled.High-concentration doping into GaInNAs may be achieved using a dopant,such as Si, Se, Te, etc.

Generally as such a waveguide clad layer, it is preferable to use ahigh-concentration doped layer formed of a group III-V compoundsemiconductor containing N and at least one element among As, P, and Sbas the group V elements.

FIG. 5 is a sectional side view showing the arrangement of a thirdembodiment of this invention's quantum cascade laser. Laser 3, shown inFIG. 5, is an InGaAs/InAlAs type quantum cascade laser (InP-QCL) thatuses InP as the semiconductor material of the substrate.

Quantum cascade laser 3 comprises an InP substrate (semiconductorsubstrate) 30, and the respective semiconductor layers of an activelayer 31, waveguide core layers 32 and 34, and waveguide clad layers 33and 35 that are formed on InP substrate 30. Also, of the side surfacesof quantum cascade laser 3, mirror surfaces, which form the opticalresonator of this laser 3, are formed on two predetermined surfaces thatoppose each other.

Active layer 31 is a semiconductor layer that is formed on InP substrate30 and generates light of a predetermined wavelength (for example, lightin the middle infrared wavelength range) by making use of intersubbandtransitions in a quantum well structure. In the present embodiment, incorrespondence to the use of InP substrate 30 as the semiconductorsubstrate as mentioned above, active layer 31 is arranged as anInGaAs/InAlAs multiple quantum well structure.

Specifically, active layer 31 is formed, by quantum well light emittinglayers (quantum well active layers) and injection layers being laminatedalternately, to have a cascade structure in which the quantum well lightemitting layers are disposed in multiple stages. The cascade structure,the intersubband transitions in the quantum well structure, etc., ofactive layer 31 are the same as those described in regard to quantumcascade laser 1 shown in FIG. 1.

For active layer 31, with the cascade structure, n-type waveguide corelayers 32 and 34, which are low-concentration doped layers, and n⁺⁺-typewaveguide clad layers 33 and 35, which are high concentration dopedlayers, are disposed to form a waveguide structure by which the lightgenerated in active layer 31 is guided inside laser 3.

At the InP substrate 30 side (lower side in the Figure) of active layer31, waveguide core layer 32 and waveguide clad layer 33 are formed inthat order from the active layer 31 side. Waveguide core layer 32 is asemiconductor layer formed of an n-type InNPAs layer and is formed to beadjacent to active layer 31 and be lattice matched with InP substrate30.

Waveguide clad layer 33 is a semiconductor layer formed of an n⁺⁺-typeInAlAs layer and is formed adjacent waveguide core layer 32 at the sideopposite the side of active layer 31. In such an arrangement, therefractive index of waveguide core layer 32 is made higher than those ofthe adjacent active layer 31 and waveguide clad layer 33. A waveguidestructure comprising core layer 32 and clad layer 33 is thus arranged.

Also at the side opposite the InP substrate 30 side (upper side in theFigure) of active layer 31, waveguide core layer 34 and waveguide cladlayer 35 are formed in that order from the active layer 31 side.Waveguide core layer 34 is a semiconductor layer formed of an n-typeInNPAs layer and is formed to be adjacent to active layer 31 and belattice matched with InP substrate 30.

Waveguide clad layer 35 is a semiconductor layer formed of an n⁺⁺-typeInAlAs layer and is formed adjacent waveguide core layer 34 at the sideopposite the side of active layer 31. In such an arrangement, therefractive index of waveguide core layer 34 is made higher than those ofthe adjacent active layer 31 and waveguide clad layer 35. A waveguidestructure comprising core layer 34 and clad layer 35 is thus arranged.

A contact layer 36 is furthermore formed at the upper side of waveguideclad layer 35. By the above, quantum cascade laser 3, in which n⁺⁺-typeInAlAs clad layer 33, n-type InNPAs core layer 32, InGaAs/InAlAs activelayer 31, n-type InNPAs core layer 34, n⁺⁺-type InAlAs clad layer 35,and contact layer 36 are laminated in that order onto InP substrate 30,is arranged. The typical thickness and carrier concentrations of therespective semiconductor layers are the same as those shown in Table 1for quantum cascade laser 1.

The effects of the quantum cascade laser of the above-describedembodiment shall now be described.

With quantum cascade laser 3 shown in FIG. 5, active layer 31, having acascade structure in which quantum well light emitting layers aredisposed in multiple stages with injection layers in between, isprovided with the waveguide structure wherein InNPAs, which is a groupIII-V compound semiconductor containing N (nitrogen) as the group Velement, is used to form waveguide core layers 32 and 34, which areadjacent active layer 31 and in which infrared light or other lightgenerated inside laser 3 is guided. With an arrangement in which corelayers 32 and 34 are formed from such a semiconductor material,waveguide loss of generated light, due to free carrier absorption insidelaser 3, is reduced.

Also, by replacing the InGaAs, used priorly in the waveguide core layersof an InP-QCL, by InNPAs, the effective refractive index of the corelayer is increased. The thickness of the waveguide core layers and cladlayers required for adequate light containment can thus be made thin.

In general, the waveguide core layers applied to a quantum cascade laserarranged using a InP substrate are preferably formed of a group III-Vcompound semiconductor, containing N and at least one element among As,P, and Sb as the group V elements, and formed so as to be latticematched with the InP substrate. By such an arrangement, the waveguideloss due to free carrier absorption can be reduced as described aboveand the core layer thickness required for light containment can bereduced.

FIG. 6 is a sectional side view showing the arrangement of a fourthembodiment of this invention's quantum cascade laser. As with laser 3shown in FIG. 5, laser 4, shown in FIG. 6, is an InGaAs/InAlAs typequantum cascade laser that uses an InP substrate.

Quantum cascade laser 4 is arranged by laminating an n⁺⁺-type InNPAsclad layer 43, n-type InNPAs core layer 32, InGaAs/InAlAs active layer31, n-type InNPAs core layer 34, n⁺⁺-type InNPAs clad layer 45, andcontact layer 36 in that order onto InP substrate 30.

This quantum cascade laser 4 has substantially the same arrangement asquantum cascade laser 3 shown in FIG. 5. However, n⁺⁺-type InNPAs layers43 and 45 are used in place of n⁺⁺-type InAlAs layers as the waveguideclad layers adjacent the waveguide core layers 32 and 34. The typicalthickness and carrier concentrations of the respective semiconductorlayers are the same as those shown in Table 1 for quantum cascade laser1.

High-concentration doped layers, having the same N-containing groupIII-V compound semiconductor as that of waveguide core layers 32 and 34as the semiconductor material, may thus be used as the waveguide cladlayers. Also by changing the waveguide clad layers from InAlAs layers toInNPAs layers, further reduction of free carrier absorption is possible.High-concentration doping into InNPAs may be achieved using a dopant,such as Si, Se, Te, etc.

Generally as such a waveguide clad layer, it is preferable to use ahigh-concentration doped layer formed of a group III-V compoundsemiconductor, containing N and at least one element among As, P, and Sbas the group V elements.

This invention's quantum cascade laser is not limited to theabove-described embodiments and various modifications are possible. Forexample, though an example of using a gas source MBE method wasindicated in regard to the method of preparing the laminated structurein the quantum cascade laser, the laminated structure may be preparedinstead by a solid-state source MBE method, in which a N (nitrogen)source is used as the plasma source, an MOCVD method, etc.

Also, though for example with the quantum cascade laser 1 shown in FIG.1, n-type GaInNAs core layers 12 and 14 are preferably lattice matchedwith GaAs substrate 10, since optical characteristics are givenpriority, some degree of lattice mismatching is tolerable as long as itis within a range that will not influence the laser characteristicsexcessively.

Also, though with the arrangement examples described above, thecomposition ranges are those in which the added amount of N in thesemiconductor crystal is approximately 1 to 2% and thus extremely low,the above-described effects are expressed more prominently by increasingthe added amount of N. Though problems in terms of crystallinity mayoccur in some cases when the added amount of N is increased, as long asit is within a range in which use as a waveguide layer is enabled, thereis no problem in increasing the added amount of N.

This invention's quantum cascade laser shall now be described further.

The arrangement of a fifth embodiment of this invention's quantumcascade laser shall now be described with reference to FIGS. 7 and 8.FIG. 7 is a sectional side view showing the arrangement of the quantumcascade laser 5 of the present embodiment. Laser 5, shown in FIG. 7, isa quantum cascade laser that uses GaAs as the semiconductor material ofthe substrate.

Laser 5 is arranged by successively laminating active layer 51,waveguide clad layer 52, waveguide clad layer 53, and contact layer 54on top of a GaAs substrate (semiconductor substrate) 50, which is dopedto a low concentration of approximately 0.5 to 4×10¹⁷ cm⁻³. Also, of theside surfaces of laser 5, mirror surfaces, which form the opticalresonator of this laser 5, are formed on two predetermined surfaces thatoppose each other.

Active layer 51 is a semiconductor layer that is formed on GaAssubstrate 50 and generates light of a predetermined wavelength (forexample, light in the middle infrared wavelength range) by making use ofintersubband transitions in a quantum well structure.

Specifically, active layer 51 is formed, by quantum well light emittinglayers (quantum well active layers) and injection layers being laminatedalternately, to have a cascade structure in which the quantum well lightemitting layers are disposed in multiple stages. That is, active layer51 has a plurality of quantum well light emitting layers and a pluralityof injection layers, respectively disposed between the plurality ofquantum well light emitting layers and forming a cascade structure. Thenumber of quantum well light emitting layers and injection layerslaminated are set suitably, and, for example, in a case where acombination of a quantum well light emitting layer and an injectionlayer is regarded as a laminated unit, 36 such laminated units arelaminated. The thickness of the laminated unit (thickness of the quantumlight emitting layer and the injection layer in the laminationdirection) is, for example, approximately 45.3 nm. However, the filmthickness of the laminated unit and the number of laminated unitslaminated are not limited to the above.

FIG. 8 is a schematic diagram showing an example of the cascadestructure and the intersubband transitions in the quantum well structureof the active layer 51. For the sake of description, in FIG. 8, amongthe laminated structure of the quantum well light emitting layers andinjection layers that form active layer 51, the cascade structure ofthree layers, that is, the two quantum well light emitting layers 502and 504 and the single injection layer 503, sandwiched between lightemitting layers 502 and 504, is shown.

As shown in FIG. 8, each of quantum well light emitting layers 502 and504 is formed of quantum well layers 510 and quantum barrier layers 511.In each of light emitting layers 502 and 504, a subband B1 and a subbandB2 are formed, for example, from localized quantum levels n=1 and 2 bythese quantum well layers 510 and quantum barrier layers 511.

Also, injection layer 503 is disposed between light emitting layers 502and 504. This injection layer 503 is formed of quantum well layers 512and quantum barrier layers 513, and quantum well layers 512 are formedto become narrower in width in the direction from light emitting layer502 to light emitting layer 504.

When a bias voltage is applied to quantum cascade laser 5, having activelayer 51 with such a conduction band quantum level structure, electrons501 become injected selectively into subband B2 of light emitting layer502. An electron 501 that is injected into subband B2 transits tosubband B1, and by this intersubband transition, light hν of awavelength corresponding to the energy gap between subbands B1 and B2 isgenerated.

Also, the electron 501 that has transited to subband B1 of quantum welllight emitting layer 502 passes from subband B1 through injection layer503 and is selectively injected into subband B2 of light emitting layer504. This electron 501 then transits from subband B2 to subband B1 inquantum well light emitting layer 504. In this process, light hν of awavelength corresponding to the energy gap between subbands B1 and B2 isgenerated. The light hν that is generated in each of quantum well lightemitting layers 502 and 504 is resonated by the optical resonator oflaser 5 and output as laser light of a predetermined wavelength. When aplurality of such quantum well light emitting layers, which generatelight by the above-described intersubband transitions in the quantumwell structure, and injection layers are laminated alternately,electrons move successively in a cascading manner among the quantum welllight emitting layers and light hν is generated in the process ofintersubband transition at each light emitting layer.

In laser 5, in correspondence to the use of GaAs substrate 50 as thesemiconductor substrate, the quantum well layers are formed of GaInNAs,which is a group III-V compound semiconductor containing As and nitrogen(N) as the group V elements, and the quantum barrier layers are formedof AlGaAs. Here, though various composition ratios may be considered forIn and N in the general formula, Ga_(x)In_(1-x)N_(y)As_(1-y) of GaInNAs,for example, the composition ratio x of In is 0.03 and the compositionratio y of N of 0.0044. By arranging the quantum well light emittinglayers and the injection layers using such a GaInNAs as the quantum welllayers, the effective refractive index of active layer 51 is madegreater than that of GaAs substrate 50.

Referring now to FIG. 7, on the side of active layer 51 opposite theGaAs substrate 50 side (on the upper side in the Figure) of active layer51, waveguide clad layer 52 and waveguide clad layer 53 are formed inthat order from the active layer 51 side. Waveguide clad layer 52 is asemiconductor layer formed of an n-type GaAs layer and has a refractiveindex that is lower than the refractive index of active layer 51.Waveguide clad layer 53 is a semiconductor layer formed of an n⁺⁺-typeGaAs layer and has an even lower refractive index than waveguide cladlayer 52.

Also as mentioned above, on top of waveguide clad layer 53 is formedcontact layer 54 for putting laser 5 in contact with an electrode forinput of current.

An example of the thickness and the carrier concentrations of therespective semiconductor layers of the above-described laser 5 are shownin Table 4.

TABLE 4 Carrier Thickness concentration Waveguide clad layer 53 1 μm n =6 × 10¹⁸ cm⁻³ Waveguide clad layer 52 3.5 μm n = 4 × 10¹⁶ cm⁻³GaInNAs/AlGaAs active 1.63 μm — layer 51 GaAs substrate 50 — n = 0.5 − 4× 10¹⁷ cm⁻³

In Table 4, active layer 51 is formed by laminating, for example, 36layers of laminated units, each comprising a quantum well light emittinglayer, formed by laminating the abovementioned quantum well layers andquantum barrier layers, and an injection layer, and in this case, thethickness (thickness in the lamination direction), order of lamination,and carrier densities of the respective layers of a laminated unit are,for example, as shown in FIG. 9. The structure of a lamination unit isnot restricted to the structure of FIG. 9.

The above-described laser 5 may for example be prepared using a gassource MBE (Molecular Beam Epitaxy) method. With this method,GaInNAs/AlGaAs active layer 51, waveguide clad layer 52, and waveguideclad layer 53 are grown successively on top of GaAs substrate 50 to formthe laminated structure of laser 5.

Here, though the AlGaAs layers, which form active layer 51, and the GaAslayers, which form waveguide clad layers 52 and 53, may be grown at aconventional growth temperature, that is, at around 600° C., the GaInNAslayers are preferably grown at approximately 450° C. in consideration ofthe phase separation of In. Also, Si may be given as an example of thedopant for waveguide clad layers 52 and 53, which are GaAs layers inwhich a dopant is implanted.

With the above-described laser 5, it is important that the quantum welllayers of active layer 51 comprise GaInNAs. Since the quantum well lightemitting layers and injection layers that form a cascade structure areformed using group III-V compound semiconductors that contain N, theeffective refractive index of active layer 51 is higher than those ofGaAs substrate 50 and waveguide clad layer 52 as mentioned above. Awaveguide structure is thus formed by GaAs substrate 50, active layer51, and waveguide clad layer 52. In other words, active layer 51 has, inaddition to the function of generating light, the function of awaveguide core layer that guides the generated light.

FIG. 10 is a diagram showing the refractive index distribution of laser5 and the light intensity distribution of the light generated at activelayer 51 in laser 5. The abscissa axis indicates the distance x to eachlayer with respect to the interface between GaAs substrate 50 and activelayer 51 in FIG. 7. The ordinate axis indicates the light intensity andthe refractive index. Solid curve I and solid curve II indicate thelight intensity distribution and the refractive index distribution,respectively. The calculation conditions are as follows. That is, it wasdeemed that the quantum well layer of active layer 51 is formed ofGa_(0.97)In_(0.03)N_(0.0044)As_(0.9956) and the quantum barrier layer isformed of Al_(0.33)Ga_(0.67)As. Also, active layer 51 was deemed to beformed by laminating 36 laminated units, each comprising a quantum welllight emitting layer, formed by laminating the abovementioned quantumwell layers and quantum barrier layers, and an injection layer. Thethickness (thickness in the lamination direction) and the order oflamination of each laminated unit were set as shown in FIG. 9.Furthermore, it was deemed that light of a wavelength of 7 μm is emittedfrom active layer 51 of the above-described arrangement.

FIG. 10 shows that the refractive index of active layer 51 in laser 5 is3.30, and thus as mentioned above, active layer 51 is higher inrefractive index than GaAs substrate 50 (refractive index: 3.23) andwaveguide clad layer 52. Thus as can be understood from solid curve I,light containment in active layer 51 is enabled. The light containmentfactor in this case is 0.35.

Here, for comparison, an arrangement example of a prior-art quantumcascade laser shall be described. FIG. 11 is a sectional side viewshowing a prior-art quantum cascade laser. Laser 8, shown in FIG. 11, isa GaAs/AlGaAs type quantum cascade laser that uses a GaAs substrate.

Laser 8 is arranged by the successive lamination of waveguide clad layer81, waveguide core layer 82, active layer 83, waveguide core layer 84,waveguide clad layer 85, and contact layer 86 on top of GaAs substrate80.

Waveguide clad layers 81 and 85 are semiconductor layers comprisingn⁺⁺-type GaAs layers and wave guide core layers 82 and 84 aresemiconductor layers comprising n-type GaAs layers. Active layer 83 hasthe same cascade structure as active layer 51 of FIG. 7. However, withactive layer 83, the quantum well layers are formed of GaAs and thequantum barrier layers are formed of AlGaAs.

An example of the carrier concentration of GaAs substrate 80 and thethickness and the carrier concentrations of the respective layers oflaser 8 are shown in Table 5.

TABLE 5 Carrier Thickness concentration Waveguide clad layer 85   1 μm n= 6 × 10¹⁸ cm⁻³ Waveguide core layer 84 3.5 μm n = 4 × 10¹⁶ cm⁻³GaAs/AlGaAs active layer 83 1.63 μm  — Waveguide core layer 82 3.5 μm n= 4 × 10¹⁶ cm⁻³ Waveguide clad layer 81 1.0 μm n = 6 × 10¹⁸ cm⁻³ GaAssubstrate 80 — n = 3 × 10¹⁸ cm⁻³

In Table 5, active layer 83 is formed by laminating, for example, 36layers of laminated units, each comprising a quantum well light emittinglayer, formed by laminating quantum well layers (GaAs) and quantumbarrier layers (AlGaAs), and an injection layer, and in this case, thethickness (thickness in the lamination direction), order of lamination,and carrier densities of the respective layers of a laminated unit are,for example, as shown in FIG. 12.

Laser 8 differs from laser 5 in that GaAs substrate 80, which is dopedto a high concentration of approximately 3×10¹⁸ cm⁻³, is used as thesemiconductor substrate, GaAs is used as the quantum well layers ofactive layer 83, and a waveguide clad layer 81 and a waveguide corelayer 82 are disposed between active layer 83 and GaAs substrate 80.

FIG. 13 is a diagram showing the refractive index distribution and thelight intensity distribution of laser 8. The abscissa axis indicates thedistance x to each layer with respect to the interface between GaAssubstrate 80 and waveguide clad layer 81 in FIG. 11. The ordinate axisindicates the light intensity and the refractive index. Solid curve IIIand solid curve IV indicate the light intensity distribution and therefractive index distribution, respectively. The calculation conditionsare as follows. That is, it was deemed that the quantum well layer ofactive layer 83 is formed of GaAs and the quantum barrier layer isformed of Al_(0.33)Ga_(0.67)As. Also, active layer 83 was deemed to beformed by laminating 36 laminated units, each comprising a quantum welllight emitting layer, formed by laminating quantum well layers andquantum barrier layers, and an injection layer. The thickness (thicknessin the lamination direction) and the order of lamination of eachlaminated unit were set as shown in FIG. 12. Furthermore, it was deemedthat light of a wavelength of 9 μm is emitted from active layer 83 ofthe above-described arrangement. The effective refractive index ofactive layer 83 is 3.21 and lower than the refractive indices of GaAswaveguide core layers 82 and 84. Thus in order to contain light inactive layer 83, waveguide clad layers 81 and 85, which are doped tohigh concentrations, are provided.

As shown in FIG. 13, since the effective refractive index of activelayer 83 is less than the refractive indices of waveguide core layers 82and 84, light is not adequately contained in active layer 83. Thus inorder to reduce the free carrier absorption in waveguide clad layers 81and 85, waveguide core layers 82 and 84 has to be made adequately thick(for example, 3.5 μm) to attenuate the light intensity before the lightreaches waveguide clad layers 81 and 85. Since the thickness of laser 8is thus increased, the serial resistance of the element is increased andhigh self-heating occurs.

On the other hand, with laser 5 of the present embodiment, since GaInNAsis used in active layer 51 as mentioned above, GaAs substrate 50 can beused as a waveguide clad layer. Waveguide clad layer 81 and waveguidecore layer 82 in laser 8 shown in FIG. 11 can thus be omitted. Since thelayer thickness above GaAs substrate 50 is thus thinned and the elementresistance is made small, self-heating of the element is reduced. CWoperation at a thermoelectrically cooled temperature, CW operation atroom temperature, as well as high output and other high performance canthus be realized in a quantum cascade laser that uses a GaAs substratethat is inexpensive and excellent in crystallinity. Furthermore, sincethere only a few layers on top of GaAs substrate 50, the arrangementenables simplification of the manufacturing process and improvement ofthe manufacturing efficiency of laser 5.

Also, normally with an interband transition laser based on electron-holerecombination, a lattice matched composition was avoided and acomposition of a high strain region was used to avoid the weakening ofcontainment with respect to the holes. This is because in the region oflattice matching with GaAs, the valence band level of GaInNAs approachesthat of GaAs and a valence band discontinuity is thus substantiallyeliminated.

With laser 5, since only transitions between conduction band subbandsare used, GaInNAs of a region of lattice matching with GaAs can be used.And since by using GaInNAs that is lattice matched with GaAs, goodheteroepitaxial growth can be achieved, laser 5 can be provided witheven better crystallinity.

Though in the above description of the arrangement of laser 5, thecomposition ratio of nitrogen in the GaInNAs used in active layer 51 isset to 0.44% as an example, the composition ratio of nitrogen may be noless than 0.1% and no more than 40% and preferably no less than 0.1% andno more than 10%.

FIG. 14 is a diagram showing the relationship of the composition ratioof nitrogen in the GaInNAs in laser 5 and the light containment factorof active layer 51. More specifically, the Figure shows the variation ofthe light containment factor (in other words, the variation of therefractive index) of active layer 51 when nitrogen is added toGa_(0.97)In_(0.03)As, with which the composition ratio of In is 0.03. Ascan be understood from FIG. 14, in the region in which the compositionratio of nitrogen is less than 0.1% (the hatched part of FIG. 14) acalculation solution does not exist and light containment is notachieved. In other words, when the composition ratio of nitrogen inGaInNAs is less than 0.1%, the waveguide mode of active layer 51 is aleakage mode and laser emission does not occur. It is thus favorable forthe composition ratio of nitrogen in GaInNAs to be no less than 0.1%. Inthe calculation, it was deemed that light of 7 μm is generated at activelayer 51.

Meanwhile, it is known that, due to the electronegativity of nitrogenbeing large in comparison to the other atoms, GaInNAs and other groupIII-N-V type mixed crystal semiconductors exhibit a very large bandbowing with respect to the mixed crystal composition. Though it ispredicted that when nitrogen is added to GaAs, the band gap willnormally become greater since GaAs will become closer to GaN of widegap, it is known that due to the very large band bowing, the band gapdecreases below that of GaAs once and then increases towards that ofGaN. According to Bellaiche et al., the band gap decreases rapidly untilthe composition ratio of nitrogen is approximately 10% and thereafterdecreases gradually and turns to increasing when the composition ratioof nitrogen becomes 40% or more (Phys. Rev. B. Vol. 54, p 17568 (1996)).

Also, GaInAs, especially GaAs, becomes reduced in band gap and alsobecomes reduced in lattice constant with the addition of nitrogen.Meanwhile, the band gap decreases and the lattice constant increaseswith the addition of In. Thus though the composition ofGa_(1-x)In_(x)N_(y)As_(1-y) that is lattice matched with GaAs can takeon various values according to In and N, since the reduction of the bandgap due to addition of nitrogen occurs until the composition ratio ofnitrogen is approximately 40%, the composition ratio of nitrogen ispreferably no more than 40%.

Though in the literature of C. Sirtori et al., it is indicated thatlaser emission at a wavelength of 9 μm occurred at 77K, with a quantumcascade laser, the shortest emission wavelength that can be achieved isdependent on the intrinsic conduction band discontinuity (ΔE_(c)) of thematerial. FIG. 15 is a schematic diagram of an example of the conductionband discontinuity between a quantum well layer W and quantum barrierlayers B in a prior-art quantum cascade laser that uses a GaAssubstrate. FIG. 15 illustrates the case where quantum well layer W isformed of GaAs and quantum barrier layers B are formed ofAl_(0.33)Ga_(0.67)As. For the arrangement shown in FIG. 15, ΔE_(c) isestimated to be 0.264 eV and if it is presumed that half of this is usedin intersubband transitions, light hν₁ of a wavelength of approximately8 μm is generated.

Priorly, the short wavelength limit of a quantum cascade laser using aGaAs substrate was 8 μm as mentioned above, and the short wavelengthlimit of a quantum cascade laser using an InP substrate was 3.5 μm.Though shortening of the wavelength of a quantum cascade laser has beenachieved with nitride materials (GaN/AlN) with a large conduction banddiscontinuity, adequate results have not been obtained since there areno satisfactory conductive substrates and there are difficulties interms of both design and manufacturing technologies.

On the other hand, shortening of the emission wavelength is enabled withthe arrangement of laser 5. FIG. 16 is a schematic diagram of an exampleof the conduction band discontinuity in the case where a quantum welllayer (for example, 510 in FIG. 8) is formed ofGa_(0.97)In_(0.03)N_(0.02)As_(0.98) and quantum barrier layers (forexample, 511 in FIG. 8) are formed of Al_(0.33)Ga_(0.67)As. Theconduction band discontinuity ΔE_(c) in this case is estimated to be0.611 eV. If as in the above-described case, half of this ΔE_(c) ispresumed to be used in intersubband transitions, light hν₂ of awavelength of approximately 4 μm is generated, thus far surpassing theshort wavelength limit of 8 μm of the prior art. Due to the use ofintersubband transitions, a quantum cascade laser has thecharacteristics of being fast in frequency response, small in relaxationoscillation, enabling multiple wavelength emission, etc. Thus ifshortening of the emission wavelength is enabled as described above,application as a communication light source is also possible.

As described above, with quantum cascade laser 5 of the arrangementshown in FIG. 7, since the quantum well layer of active layer 51 isformed of nitrogen-containing GaInNAs, the effective refractive index ofactive layer 51 is greater than that of GaAs substrate 50. Since in thiscase, GaAs substrate 50 can be made to function as a clad layer, activelayer 51 can be formed adjacently on top of GaAs substrate 50. The layerthickness above the semiconductor substrate of laser 5 is thus madethinner than that of the prior art, the element resistance of laser 5 isreduced, and self-heating by laser 5 is restrained. Laser 5 can thus bemade high in output and continuous (CW) emission at high temperature isalso enabled.

FIG. 17 is a sectional side view showing the arrangement of a sixthembodiment of this invention's quantum cascade laser. As with laser 5shown in FIG. 7, laser 6, shown in FIG. 17 is a quantum cascade laserthat uses a GaAs substrate.

Laser 6 is arranged by successively laminating an n-type GaAs layer 61,GaInNAs/AlGaAs active layer 51, n-type GaAs layer 62, waveguide cladlayer 53, and contact layer 54 on top of a GaAs substrate 60 that isdoped to a high concentration of approximately 3×10¹⁸ cm⁻³. As withlaser 5 of the fifth embodiment, of the side surfaces of laser 6, mirrorsurfaces, which form the optical resonator of this laser 6, are formedon two predetermined surfaces that oppose each other. An example of thecarrier concentration of GaAs substrate 60 and the thickness and thecarrier concentrations of the respective layers of laser 6 are shown inTable 6. The arrangement of active layer 51 is the same as that of thefifth embodiment and, in the present example, 36 laminated units of thearrangement shown in the table of FIG. 9 are laminated.

TABLE 6 Carrier Thickness concentration Waveguide clad layer 53 1 μm n =6 × 10¹⁸ cm⁻³ Waveguide core layer 62 3.5 μm n = 4 × 10¹⁶ cm⁻³GaInNAs/AlGaAs active layer 51 1.63 μm — Waveguide core layer 61 1 μm n= 4 × 10¹⁶ cm⁻³ GaAs substrate 60 — n = 3 × 10¹⁸ cm⁻³

Laser 6 differs from laser 5 in that GaAs substrate 60, which is dopedto a high concentration of approximately 3×10¹⁸ cm⁻³, is used as thesemiconductor substrate and an n-type GaAs layer 61 is disposed betweenactive layer 51 and GaAs substrate 60.

Laser 6 of the above-described arrangement also has active layer 51 thatuses GaInNAs in the quantum well layers and the effective refractiveindex of active layer 51 is greater than those of GaAs substrate 60 andGaAs layers 61 and 62. Thus there is no need to provide waveguide cladlayer 81 between active layer 51 and GaAs substrate 60 as was the casein laser 8 of FIG. 11 and GaAs layer 61 can thus be made thin. Since thelayer thickness above GaAs substrate 60 can thus be made thin, theelement resistance can be reduced as in the fifth embodiment. Also aswith the fifth embodiment, the emission wavelength of laser 6 can beshortened in comparison to prior-art quantum cascade lasers that use aGaAs substrate.

FIG. 18 is a diagram showing the light intensity distribution and therefractive index distribution of laser 6 of the present embodiment. Thecalculation conditions are the same as those of the fifth embodiment.The abscissa axis indicates the distance x to each layer with respect tothe interface between GaAs substrate 60 and GaAs layer 61 in FIG. 17.The ordinate axis indicates the light intensity and the refractiveindex. Solid curve V and solid curve VI indicate the light intensitydistribution and the refractive index distribution, respectively. As canbe understood from FIG. 18, n-type GaAs layer 61, which lies on top ofGaAs substrate 60 and between GaAs substrate 60 and active layer 51,serves as core layer along with GaAs layer 62 at the opposite sideacross active layer 51.

FIG. 19 is a diagram showing the relationship of the thickness of GaAslayer 61 and the light containment factor. The abscissa axis indicatesthe thickness of GaAs layer 61 and the ordinate axis indicates the lightcontainment factor. As can be understood from FIG. 19, the lightcontainment factor increases monotonously in the thickness range of 0.0μm to 2.0 μm of GaAs layer 61 and decreases when the thickness exceeds2.0 μm. The thickness of GaAs layer 61 is thus preferably no more than2.0 μm.

Also, when the thickness of GaAs layer 61 is close to 1.0 μm, the lightcontainment factor is approximately 0.40 and when the thickness is closeto 2.0 μm, the light containment factor is approximately 0.43. As canthus be understood, the provision of GaAs layer 61 and suitableadjustment of its thickness enables reinforcement of the effect ofcontaining light in active layer 51.

FIG. 20 is a sectional side view showing the arrangement of a seventhembodiment of this invention's quantum cascade laser. As with laser 5shown in FIG. 7, laser 7, shown in FIG. 20 is a quantum cascade laserthat uses a GaAs substrate.

Laser 7 differs from laser 6 of the sixth embodiment in that undopedGaInNAs layer 71 and 72 are disposed adjacent active layer 51 andrespectively between GaAs layer 61 and active layer 51 and betweenactive layer 51 and GaAs layer 62. The typical thickness of each ofGaInNAs layers 71 and 72 is, for example, 0.3 μm.

With the above-described arrangement, the respective layers between GaAslayer 61 and GaAs layer 62, that is, GaInNAs layer 71, active layer 51,and GaInNAs layer 72 function as a whole as a core layer and form awaveguide structure along with GaAs substrate 60 and waveguide cladlayer 53.

Though the composition of the abovementioned GaInNAs layers 71 and 72 isthe same as the composition of the GaInNAs used in active layer 51, therefractive index of GaInNAs layers 71 and 72 is higher than theeffective refractive index of GaInNAs/AlGaAs active layer 51. Theeffective refractive index of the core layer, formed of the respectivelayers between the abovementioned GaAs layers 61 and 62, is thus madelarge. The light containment can thereby be strengthened. Since theeffective refractive index of the core layer, formed of active layer 51and GaInNAs layers 71 and 72 is greater than those of GaAs substrate 60and GaAs layers 61 and 62, GaInNAs layers 71 and 72 may be made thin, asexemplified by the typical thickness (0.3 μm) of each of GaInNAs layers71 and 72. The layer thickness of the entirety above GaAs substrate 60is thus made thinner than that of the prior-art laser 8 shown in FIG. 11and the element resistance can be reduced. Also, as in the case of thefifth embodiment, the emission wavelength of the above-described laser 7can be made shorter than in the prior-art quantum cascade laser using aGaAs substrate.

FIG. 21 is a diagram showing the light intensity distribution and therefractive index distribution of laser 7. The calculation conditions arethe same as those of the fifth embodiment. The abscissa axis and theordinate axis are the same as those of FIG. 18. Solid curve VII andsolid curve VIII in FIG. 21 indicate the light intensity distributionand the refractive index distribution, respectively. As can beunderstood from FIG. 21, the layers made up of the respective layersbetween GaAs layers 61 and 62 function as a whole as a core layer.

Though preferred embodiments of this invention have been describedabove, this invention is obviously not limited to the above-describedembodiments. For example, though the quantum well layers in active layer51 are formed of GaInNAs, it is sufficient that these be formed of agroup III-V compound semiconductor, containing nitrogen (N) and at leastone element among As, P, and Sb as the group V elements.

Also, though an example of using a gas source MBE method was indicatedin regard to the method of preparing the laminated structure in thequantum cascade laser, the laminated structure may be prepared insteadby a solid-state source MBE method, in which a N (nitrogen) source isused as the plasma source, an MOCVD method, etc.

Also, though for example with laser 5 shown in FIG. 7, active layer 51is preferably lattice matched with GaAs substrate 50, since opticalcharacteristics are given priority, some degree of lattice mismatchingis tolerable as long as it is within a range that will not influence thelaser characteristics excessively.

Furthermore, though a GaAs substrate doped to a low concentration isused as the GaAs substrate in the fifth embodiment, an undoped GaAssubstrate may be used instead. Also, though a GaAs substrate doped to ahigh concentration is used as the GaAs substrate in each of the sixthand seventh embodiments, a GaAs substrate doped to a low concentrationmay be used instead. Also, though with the seventh embodiment, GaInNAslayers 71 and 72 are disposed at the respective outer sides of activelayer 51 in the laminated structure on GaAs substrate 60, the GaInNAslayers do not have to be provided at both sides necessarily and theselayers furthermore do not have to be GaInNAs layers. For example, it issufficient that a semiconductor layer, formed of a group III-V compoundsemiconductor, containing nitrogen and at least one element among As, P,and Sb as the group V elements, be disposed adjacent active layer 51 atleast either between active layer 51 and GaAs substrate 60 or at theside of active layer 51 opposite the GaAs substrate 60 side.

Also, though in the description of the cascade structure of active layer51, it was deemed for example that two subbands B1 and B2 are formed,this invention is not limited to cases in which two subbands are formed.That is, three or more subbands may be formed by changing the filmthickness, etc., of the quantum well layers and quantum barrier layers.Light of different wavelengths can be generated by making use ofdifferent intersubband transitions in such cases.

As has been described in detail above, this invention's quantum cascadelaser provides the following effects. That is, by an arrangement inwhich an active layer, formed on a GaAs or InP semiconductor substrateand having multiple stages of quantum well light emitting layers withinjection layers in between, is provided with a waveguide structurewherein waveguide core layers, by which infrared light or other lightgenerated inside a laser is guided, are formed using a group III-Vcompound semiconductor containing N as the group V element, waveguideloss due to free carrier absorption of the generated light is reduced.Also with this waveguide structure, the effective refractive index ofthe waveguide core layer is increased. The thickness of the waveguidecore layers and clad layers that are required for light containment canthus be made thin.

Also, with this invention's quantum cascade laser, by using a groupIII-V compound semiconductor, containing nitrogen and at least oneelement among As, P, and Sb as the group V elements, in the activelayer, the effective refractive index of the active layer can be madehigher than that of the GaAs substrate. The semiconductor substrateformed of GaAs can thus be made to function as a clad layer and thethickness of the element can thereby be made thin. The elementresistance is thereby reduced and self-heating can be restrained. Also,the layer structure between the semiconductor substrate formed of GaAsand the active layer can be simplified.

Furthermore with this invention's quantum cascade laser, shortening ofthe emission wavelength can be realized. The short wavelength limit,which was priorly limited to the middle infrared range, can thereby beextended to the near infrared range.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims.

1. A quantum cascade laser comprising: an active layer, having a cascadestructure, in which quantum well light emitting layers and injectionlayers are laminated alternately on a semiconductor substrate formed ofInP, and generating light by intersubband transitions in a quantum wellstructure; a waveguide core layer, formed adjacent said active layer;and a waveguide clad layer, formed adjacent said waveguide core layer atthe side opposite the side of said active layer; and wherein saidwaveguide core layer is formed of a group III-V compound semiconductor,containing, as the group V elements, N and at least one element selectedfrom the group consisting of As, P, and Sb, and formed so as to belattice matched with said semiconductor substrate; wherein saidwaveguide clad layer contains a high-concentration doped layer formed ofa group III-V compound semiconductor, containing, as the group Velements, N and at least one element selected from the group consistingof As, P, and Sb.
 2. The quantum cascade laser according to claim 1,wherein said waveguide core layer is formed to a predetermined thicknessthat is set so that optical modes of higher orders will not be guided.